For some time the use of propellers that allow navigation in reverse drive have been known, and in particular propellers that can be positioned alternately on a first pitch suitable for forward navigation and a second pitch suitable for reverse navigation The propeller blades are positioned at this angle when the direction of rotation of the propulsor is inverted with respect to the direction of rotation that allows forward motion of the watercraft.
For example, the document IT1052002, by Massimiliano Bianchi, teaches how to produce a propeller in particular for use in sailing boats, in which the drive shaft and the propeller casing are mutually coupled through two teeth coplanar and orthogonal with the axis of this propeller.
When the propeller is stationary, the blades are in “feathered” position, in a manner such as to generate minimum resistance, and the teeth of the drive shaft and of the propeller casing are mutually spaced apart in a manner such that subsequent driving in rotation of the drive shaft, both in one direction and the other, causes its idle rotation for a given angular range, which, due to an appropriate kinematic mechanism with pinion and gear wheels, corresponds to rotation of the blades with respect to the cylindrical casing.
When the drive shaft reaches the position of contact against the propeller casing, their relative rotation is inhibited, the blades are positioned with a predetermined fluid dynamic pitch.
In this manner, the propeller blades can reach a first pitch, and therefore a given angle of incidence, suitable for forward motion of the watercraft and a second pitch, suitable for reverse motion of the watercraft, depending on the direction of rotation of the drive shaft with respect to the propeller casing.
However, with a propeller of the type described above it is not possible to obtain a discrete or continuous variation of the fluid dynamic pitch when the operating conditions of the propeller vary.
That is, once the most appropriate pitch of the blades for forward drive and the most appropriate pitch for reverse drive of the watercraft has been established during the design stage, it is no longer possible for the operator to vary this angle of rotation in order to vary the pitch during operation of the propeller.
In other words, with this type of propeller it is possible only to reach a single pitch for forward motion and one for reverse motion, both established in advance during the design stage.
For this reason, when they are installed on different propulsors with respect to those for which they were designed, this causes a reduction in propeller performances in terms of efficiency.
In fact, determination of the optimal propeller pitch for forward drive and for reverse drive, is also obtained in relation to the characteristics of the propulsor, and in particular the torque delivered and the rotation speed that can be reached. In this case, modification of the propeller pitch can only be obtained by disassembling the propeller and taking action on the inside thereof with replacement of the hub or of the propeller casing, or by subjecting these elements to mechanical machining operations.
Only by performing these operations relative rotation of the drive shaft with respect to the propeller casing take the blades to be positioned with the desired pitch depending on both installation and using requirements. Naturally, the user of the propeller is not able to disassemble the propeller and replace or mechanically machine its parts alone and for this reason the work of a skilled operator is necessary or the propeller must be sent to the manufacturer.
Propellers in which the fluid dynamic pitch variation of the blades takes place automatically, by means of the propeller drive, are also known in the art. Generally these propellers comprise a cylindrical propeller casing, on which the propeller blades are pivoted according to a direction transverse to the axis of the propeller casing, or more generally perpendicular to the forward axis of the propeller, and a drive shaft, coupled coaxially to the propeller casing.
The propeller is also provided with means for transmitting rotary motion from the shaft to the propeller casing, and with a kinematic mechanism for adjusting the rotary motion of each blade about its axis of pivoting to the propeller casing, preferably adapted to transform the rotary motion of the drive shaft into a rotary motion of each blade about its axis of pivoting.
In order to allow operation of the aforesaid kinematic mechanism to transform rotation of the drive shaft into rotation of the blades, the motion transmission means allow the shaft to rotate idly with respect to the propeller casing at least for a predefined angular range. Idle rotation of the drive shaft in this angular range, with respect to the propeller casing, causes, due to the aforesaid kinematic mechanism for adjustment/transformation, relative rotation of the blades with respect to the propeller casing, with consequent variation in their angle of incidence with respect to the fluid and therefore of the fluid dynamic pitch.
A propeller of this type is described in the document WO 2008/075187, by Max Prop S.r.l., in which relative rotation of the drive shaft with respect to the propeller casing is adjusted by an elastic element interposed therebetween, which allows continuous pitch adjustment during operation.
In particular, the elastic element allows the blades to be positioned to the optimal pitch during operation, balancing the forces acting on the propeller, mainly the drive torque generated by the propulsor and the drag torque, until reaching a balanced position.
Although ensuring high performances in terms of efficiency during forward motion of the watercraft, this type of propeller cannot be used for navigation in reverse drive, as the value of the pitch in reverse drive is linked to the value of the pitch in forward drive. In other words, the two pitches are not independent from each other and for this reason it is not possible to adjust the pitch for navigation in reverse drive as this modification would cause a variation in the pitch for forward drive with non optimal values.
An object of the present invention is to provide a variable pitch propeller with automatic pitch adjustment during forward drive that does not have the limitations and drawbacks of prior art described above and that can also be used for navigation in reverse drive.
Another object of the present invention is to provide a propeller that allows automatic pitch adjustment during use in forward drive and, at the same time, allows modification of the pitch for navigation in reverse drive by the user, in a rapid and simple manner, without the need to disassemble the propeller or subject the internal parts thereof to mechanical machining operations.